Cathode ray tubes (CRTs) utilized in projection systems are generally high brightness CRTs requiring magnetic focus instead of the traditional electrostatic focus used in other CRT based systems. The use of magnetic focus makes it possible to achieve smaller spot size and hence higher resolution on the face plate of the CRT even though the electron beam diameter within the CRT neck is large for a projection CRT.
In prior art magnetic focus systems the magnetic focus coil is typically connected directly to the output of the focus amplifier circuit.
A significant drawback of utilizing magnetic focus coils in projection CRTs is the requirement for substantial power from the drive circuits in order to effect proper focusing across the flat CRT face plate.
Furthermore, since a video projection system must often be used with different sources of video information, allowances must be made for the various horizontal scanning rates provided by such diverse video information sources. For example, the well known video cassette recorder (VCR) operates at a horizontal scanning rate of 15.75 kHz, whereas a SUN.TM. work station operates at 64 kHz, and a CHROMATICS.TM. work station operates at a horizontal scanning rate of 126.9 kHz. In general, higher frequencies of horizontal scanning rate provide higher resolution pictures.
In such cases where variable scan rates are provided to which the dynamic focus correction must be synchronized, a linear focus amplifier/drive circuit is necessary to implement focusing over the required scan rate range.